Pulse trap makes optical switch

By
Kimberly Patch,
Technology Research NewsScientists who work with light pulses so
short that one trillion of them pass by in a second are laying the groundwork
for higher bandwidth communications and blazingly-fast, all-optical computer
chips.

But making the most of these ridiculously short pulses requires
that they be switched on and off using other light pulses rather than
electronic devices. Researchers have been working on all-optical switching
for decades; the challenge is finding a fast, efficient method that uses
little power.

Researchers from Nagoya University in Japan have found a way to
capture an ultrashort pulse by using a second pulse to filter out the
first one as it travels through an optical fiber. Selectively knocking
out pulses from a string, or train, of pulses makes for a sequence of
pulses and gaps that can represent the ones and zeros of digital information.
Using light pulses to do this means that, like electronic circuits, the
output of one logic unit can control another.

The method takes advantage of a phenomenon known as optical pulse
trapping -- meaning one optical pulse can overlap with and travel with
another. "The trapped pulse spatially overlaps with [a] control pulse
and they copropagate along the fiber," said Norihiko Nishizawa, an assistant
professor of quantum engineering at Nagoya University.

The researchers demonstrated the method by sending a train of
signal pulses down an optical fiber, then picking off one of the pulses
in the train by making a control pulse overlap the targeted signal pulse.
"We can trap [a single] pulse from high-repetition-rate pulse trains,"
said Nishizawa. The method is nearly 100 percent efficient, he said.

As the pulses propagate down a fiber optic line, the waveform
of the trapped pulse is compressed into a shorter wavelength. The control
pulse is a soliton, a type of wave that doesn't normally spread out as
it travels. Solitons can be made to shift to a longer wavelength, and
when a soliton that is trapping another pulse shifts, the trapped pulse
is forced to shift to a shorter wavelength to compensate.

The shift makes it possible to identify and pick off only the
trapped pulses, said Nishizawa. "Since the wavelength of the trapped pulse
is distinctly shifted and separated from the other untrapped pulses, we
can pick off only the trapped pulse easily using [a] wavelength filter,"
he said.

The researchers used an ultra-fast optical pulse device that they
demonstrated in 1999 to provide light pulses separated by only 1.5 picoseconds,
or trillionths of a second. A trillionth of a second is to a second as
a second is to 31,709 years.

This is equivalent to a communications rate of .67 terahertz,
or trillion bits per second, according to Nishizawa. The method could
make it possible to use pulses separated by one picosecond to provide
a communications speed of one terahertz, he said. Today's high-speed communications
equipment uses tenth of a nanosecond pulses to provide top communications
speeds of 10 billion bits per second per channel.

The researchers modified a crosscorrelation frequency resolved
optical grating (X-FROG) system to measure the ultrashort optical pulses
so they could prove that the system worked. The test system provided the
researchers with spectrograms that showed the wavelength of light passing
through at any given trillionth of a second. "We have developed a highly
sensitive X-FROG system so that we could directly observe the ultrafast
all-optical switching," said Nishizawa.

The pulse trapping method could eventually be used in ultrafast
optical communications, and optical information processing, said Nishizawa.

The idea of using soliton trapping gates is not new, but the researchers
are using a different physical effect -- the compensating shift of the
trapped pulse -- to filter out pulses, said Curtis Menyuk, a computer
science and electrical engineering professor at the University of Maryland
Baltimore County.

The method is one of a large number of proposed all-optical switching
schemes, said Menyuk. The Nagoya method is relatively intricate and delicate,
however. Its drawbacks are that the range of allowed powers and frequencies
is small, and the approach doesn't make it possible to cascade more than
one device, he said.

The researchers' next step is demonstrating the method at the
1.55 micron wavelength region used for long distance optical communications.

Nishizawa's research colleague was Toshio Goto. The work appeared
in the February 24, 2003 issue of Optics Express. The research
was funded by the Japanese Ministry of Education, Science, Sports and
Culture.